1. Field of the Invention
Embodiments of invention relate to a method for fabricating an oxide thin film transistor, and more particularly, to a method for fabricating an oxide thin film transistor using a ternary system or quaternary system oxide semiconductor comprising a combination of AxByCzO (A, B, C=Zn, Cd, Ga, In, Sn, Hf, Zr; x, y, z≧0) as an active layer.
2. Description of the Related Art
As interest in information displays is growing and demand for portable (mobile) information medium is increasing, research and commercialization of lighter and thinner flat panel displays (“FPD”), which may substitute for cathode ray tubes (CRTs), the theretofore conventional display devices, have been actively ongoing. Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystals. LCD devices exhibit excellent resolution, color display and picture quality, so they are commonly used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal of a pixel part is driven by using an amorphous silicon thin film transistors (a-Si TFTs) as a switching element.
The structure of a related art LCD will now be described in detail with reference to FIG. 1. FIG. 1 is an exploded perspective view showing a related art LCD device. As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for demarcating the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying a voltage to the liquid crystal layer 30.
The array substrate 10 includes a plurality of gate lines 16 and a plurality of data lines 17 which are arranged vertically and horizontally to define a plurality of pixel regions (P), TFTs (T), switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel regions (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant formed at the edges of an image display area to form a liquid crystal panel, and the attachment of the color filter substrate 5 and the array substrate 10 is made by attachment keys formed on the color filter substrate 5 or the array substrate 10.
The above-described LCD is a display device which has come into prominence so far with its advantages of being light and consuming a small amount of power, but it is a light receiving device, not a light emitting device, and has a technical limitation with respect to brightness, a contrast ratio, a viewing angle, and the like. Thus, development of a new display device that may overcome such shortcomings is actively ongoing.
An organic light emitting diode (OLED), one of the new types of flat panel display devices, is self-emissive, having a good viewing angle and contrast ratio compared with the LCD. Because it does not require a backlight, it can be formed to be lighter and thinner, and is advantageous in terms of power consumption. Also, the OLED is driven at a low DC voltage and has a fast response speed. In particular, the OLED is advantageous in terms of fabrication costs.
Recently, research for a large-scale OLED has been actively ongoing, and in order to attain it, development of a transistor achieving a stable operation and durability by securing constant current characteristics of a driving transistor of the OLED is required.
The amorphous silicon thin film transistor used for the above-described LCD may be fabricated in a low temperature process, but has a very small mobility and does not satisfy constant current bias conditions. Meanwhile, a polycrystalline silicon thin film transistor has high mobility and satisfies the constant current bias conditions, but it is difficult to obtain uniform characteristics, making it difficult to increase the size and requiring a high temperature process.
Thus, an oxide semiconductor thin film transistor including an active layer made of oxide semiconductor has been developed, but when the oxide semiconductor is applied to the thin film transistor having a typical bottom gate structure, the oxide semiconductor is damaged during an etching process of source and drain electrodes, in particular, during a dry etching process using plasma.
Thus, in order to address the problem, an etch stopper is additionally formed as a barrier layer on the active layer, but even in this instance, a back channel region of the active layer is exposed to the chemical material such as a photoresist or a stripper, and ultraviolet (UV) ray used for a photolithography process (or a photo process) for forming the active layer and the etch stopper, changing the characteristics of the oxide semiconductor to degrade the element characteristics.
FIG. 2 is a sectional view schematically showing a structure of the related art oxide thin film transistor. As illustrated, the related art oxide thin film transistor includes a gate electrode 21 formed on a substrate 10, a gate insulating layer 15a formed on the gate electrode 21, an active layer 24 formed of an oxide semiconductor and an etch stopper 25 made of a certain insulating material and formed on the gate insulating layer 15a, source and drain electrodes 22 and 23 electrically connected with certain areas of the active layer 24, a protective film 15b formed on the source and drain electrodes 22 and 23, and a pixel electrode 18 electrically connected with the drain electrode 23.
FIGS. 3A to 3F are sectional views sequentially showing a process of fabricating the related art oxide thin film transistor illustrated in FIG. 2. As shown in FIG. 3A, a first conductive film is deposited on the entire surface of a certain substrate 10 and selectively patterned through photolithography to form the gate electrode 21 formed of the first conductive film on the substrate 10.
Next, as shown in FIG. 3B, the gate insulating layer 15a and an oxide semiconductor layer made of a certain oxide semiconductor are sequentially deposited on the entire surface of the substrate 10, and then selectively patterned through photolithography to form the active layer 24 formed of the oxide semiconductor on the gate electrode 21.
Then, as shown in FIG. 3C, an insulating layer made of a certain insulating material is deposited on the entire surface of the substrate 10 and then selectively patterned through photolithography to form an etch stopper 25 made of the insulating material on the active layer 24.
Thereafter, as shown in FIG. 3D, a second conductive film is formed on the entire surface of the substrate 10 having the etch stopper 25 formed thereon and then selectively patterned through photolithography to form source and drain electrodes 22 and 23 formed of the second conductive film and electrically connected with source and drain regions of the active layer 24.
Then, as shown in FIG. 3E, the protective film 15b is formed on the entire surface of the substrate 10 having the source and drain electrodes 22 and 23 formed thereon and then selectively patterned through photolithography to form a contact hole 40 exposing a portion of the drain electrode 23.
Thereafter, as shown in FIG. 3F, a third conductive film is formed on the entire surface of the substrate 10 and then selectively patterned through photolithography to form the pixel electrode 18 electrically connected with the drain electrode 23 through the contact hole 40.
Namely, in the related art, after the oxide semiconductor layer is deposited, the active layer 24 is formed through photolithography, and then, the insulating layer is deposited to form the etch stopper 25. Also, the insulating layer is patterned through another photolithography process to form the etch stopper 25.
In this instance, the patterning of the active layer 24 and the depositing of the insulating layer are performed after a vacuum state of a vacuum chamber is released, so the oxide semiconductor is exposed in the air, and also, as the photolithography process is performed to pattern the active layer 24 and the etch stopper 25, the back channel region is exposed to the chemical material such as the photoresist or a stripper and UV, so as to be damaged. As a result, the electrical characteristics of the thin film transistor are degraded.
In general, the oxide semiconductor has two kinds of characteristics of a conductor and a semiconductor and may transition therebetween by adjusting carrier concentration within the thin film. The carrier concentration can be adjusted by electrons created as oxygen vacancies are generated, and in this instance, the oxygen vacancies are generated due to the damage of the oxide semiconductor in various processes. Results of research show that the oxide semiconductor is also damaged even by a solvent of a base besides generally known acid.